Auto merge of #128812 - nnethercote:shrink-TyKind-FnPtr, r=compiler-errors

Shrink `TyKind::FnPtr`.

By splitting the `FnSig` within `TyKind::FnPtr` into `FnSigTys` and `FnHeader`, which can be packed more efficiently. This reduces the size of the hot `TyKind` type from 32 bytes to 24 bytes on 64-bit platforms. This reduces peak memory usage by a few percent on some benchmarks. It also reduces cache misses and page faults similarly, though this doesn't translate to clear cycles or wall-time improvements on CI.

r? `@compiler-errors`

compiler/rustc_mir_transform/src/shim.rs

@@ -437,7 +437,7 @@ fn build_clone_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, self_ty: Ty<'tcx>) -
     let src = tcx.mk_place_deref(Place::from(Local::new(1 + 0)));
 
     match self_ty.kind() {
-        ty::FnDef(..) | ty::FnPtr(_) => builder.copy_shim(),
+        ty::FnDef(..) | ty::FnPtr(..) => builder.copy_shim(),
         ty::Closure(_, args) => builder.tuple_like_shim(dest, src, args.as_closure().upvar_tys()),
         ty::CoroutineClosure(_, args) => {
             builder.tuple_like_shim(dest, src, args.as_coroutine_closure().upvar_tys())